3.1 Diffusion and osmosis
Water as a solvent
Water is the solvent in living things, because many substances dissolve in it. This is important in:
- digestion: digested food molecules dissolve in water and can be absorbed;
- excretion: urea and excess salts dissolve in water and are carried out in urine;
- transport: substances such as glucose, ions and hormones dissolve in the water of blood plasma (and sap in plants) and are carried around the body.
Diffusion
Diffusion is a passive process: it needs no energy from respiration. The energy comes from the kinetic energy of the random movement of the molecules and ions.
Examples: oxygen diffusing from the air in the alveoli into the blood; carbon dioxide diffusing into a leaf for photosynthesis; digested food diffusing from the small intestine into the blood.
Factors that affect the rate of diffusion.
| Factor | Effect on the rate |
|---|---|
| surface area | a larger surface area gives a faster rate |
| temperature | a higher temperature gives a faster rate, because the particles have more kinetic energy |
| concentration gradient | a steeper gradient gives a faster rate |
| distance | a shorter distance gives a faster rate |
Investigation. Agar jelly cubes that contain an indicator can be placed in acid to show how fast the acid diffuses in. Smaller cubes (a larger surface area compared with their volume) change colour all the way through faster. Another way is to time how long it takes for the colour of a dye to spread through water at different temperatures.
Osmosis
- A dilute solution has a high water potential (a lot of water). A concentrated solution has a low water potential.
- A partially permeable membrane lets small molecules such as water through, but not larger solute molecules.
- Water moves down the water potential gradient, from the dilute solution to the concentrated solution.
- Osmosis is a type of diffusion and, like diffusion, uses the kinetic energy of the molecules.
Investigating osmosis with dialysis (Visking) tubing. A tube is filled with a concentrated sugar solution, tied at both ends and placed in a beaker of water. Water moves into the tube by osmosis, so the tube becomes swollen and heavier. If the tube contains water and is placed in sugar solution, it loses mass.
Osmosis in plant tissue
Pieces of potato or other plant tissue can be placed in solutions of different concentrations. They are weighed or measured before and after.
| Solution | Water movement | Result |
|---|---|---|
| pure water or a dilute solution (higher water potential than the cells) | water moves into the cells | the tissue gains mass and becomes firm |
| concentrated solution (lower water potential than the cells) | water moves out of the cells | the tissue loses mass and becomes soft |
| a solution with the same water potential | no net movement | no change in mass |
Plotting percentage change in mass against concentration gives a graph that crosses zero where the solution has the same water potential as the cells.
Terms for plant cells.
- Turgid: a cell that is full of water, with the cell membrane pushing against the cell wall.
- Turgor pressure: the pressure of the water inside the cell pressing outwards on the cell wall. It supports plants (non-woody parts keep their shape because their cells are turgid).
- Flaccid: a cell that has lost some water, so it is soft and limp. If a plant loses enough water, it wilts.
- Plasmolysis: when so much water leaves that the cytoplasm and membrane pull away from the cell wall.
The cell wall prevents a plant cell from bursting when it takes in water, but animal cells have no cell wall. In pure water, an animal cell such as a red blood cell takes in water and may burst. In a concentrated solution, it loses water and shrinks.
Solution X has a higher water potential (more dilute) than the potato cells, so water moved in by osmosis. Solution Y has a lower water potential (more concentrated), so water moved out.
Importance in plants. Root hair cells have a lower water potential than the soil water, so water enters them by osmosis. Water then moves from cell to cell across the root down a water potential gradient.
3.2 Active transport
| Diffusion | Active transport | |
|---|---|---|
| direction | down the concentration gradient | against the concentration gradient |
| energy from respiration | not needed | needed |
| membrane proteins | not needed | carrier proteins pump the substance |
Importance in ion uptake by root hair cells. The concentration of mineral ions such as nitrate in the soil is lower than in the root hair cells, so the ions cannot enter by diffusion. Root hair cells take them up by active transport, using energy from respiration. The cells have many mitochondria to supply the energy. This lets plants take in the ions they need even when the soil contains very little of them.
Active transport also takes glucose from the small intestine and the kidney tubules into the blood when the concentration in the blood is higher.
Practise this chapter
Every past-paper question on these topics, with the mark scheme.